In order to make accurate movements throughout our lifetime, it is thought that our brain forms internal models of our body and our environment. These internal models adapt as our body and the environment that we interact with undergo changes. One type of theoretical internal model is a forward model (FM), which makes predictions about the sensory consequences of our motor commands, and provides us with prediction errors to drive adaptation. Though FMs are used computationally to model motor adaptation, there have been few experiments that have attempted to find the neural basis of FMs in the brain. Furthermore, during adaptation, we currently have no method with which to estimate how much of the change in the motor output from trial to trial is due to adaptation of FMs. Here, we present a new method with which to quantify adaptation of FMs on a trial-by-trial basis during reaching. Using this new method, we can look for the neural substrates of FMs by testing patient populations. Studies implicate the cerebellum as the most likely location for FMs, and demonstrate that cerebellar damage leads to deficits in adaptation. Recently we discovered that cerebellar subjects are able to change their motor output in response to a gradually introduced perturbation. If FMs depend on the cerebellum, then there should be scenarios in which cerebellar subjects show normal changes in motor output, with abnormal changes in FMs. Our new experimental method will allow us to test this prediction. During development, the cerebellum matures more slowly than the cerebrum. This is suspected to be the underlying reason for why young children show abnormal patterns of locomotor adaptation, resembling that of cerebellar subjects. This runs against the common belief that young children are better learners than adults. We hypothesize that with development of the cerebellum, adaptation relies more strongly on FMs. Testing these groups, in addition to healthy controls, will provide a significant new picture of FMs - how they affect our movements, their biological underpinnings, and how they come to exist in our brains.